Lenticular arrays are used to give images an appearance of depth. More specifically, a lenticular array comprises a transparent upper layer A having narrow, parallel lenticules (cylindrical lenses) B on an outer surface, and an image-containing substrate layer C. (See FIG. 1A). The image on the substrate layer C is called a composite image. It is formed by a set of image lines D. The image, viewed by the observer who is looking at a lenticular array, is called a lenticular image. The two layers of a lenticular array provide the lenticular image such that different views of this lenticular image are selectively visible as a function of the angle from which the lenticular array is viewed. If the viewed image is made by bringing together into a single composition a number of different parts of a scene photographed from different angles, and the lenticules are vertically oriented, each eye of a viewer will see different elements and the viewer will interpret the net result as a three dimensional (3-D) image. The viewer may also move his head with respect to the lenticular array, thereby observing other views with each eye and enhancing the sense of depth.
Another method for showing 3-D images is the use of a parallax media such as a blocking line screen (FIG. 1B) positioned at a specific distance from the image lines forming a composite image. This process, known as a parallax process, causes blocking of all image lines except those corresponding to one specific image. This allows the viewer's eyes to view different images as three-dimensional (3-D) images, when the blocking line screen is oriented vertically.
When a lenticular array or a blocking line screen is oriented horizontally, each eye receives the same image. In this case, the multiple images can give illusion of motion when the composite image, located on the back of a lenticular array or a blocking line screen, is rotated about a line parallel to the viewer's eyes. Thus, a simulation of motion is achieved by the process of either tipping a lenticular array or a blocking line screen containing the composite image, or by movement of the viewer's head to a different angle with respect to the lenticular array or a blocking line screen.
Whether the lenticules or the blocking line screen is oriented vertically or horizontally, each of the viewed images is generated by the image lines which have been interlaced at the spatial frequency of the lenticular array (determined by the lenticule's width) or the blocking line screen. Interlacing lines of each image with other images is referred to as interdigitation. A full set of such interdigitated image lines forms the composite image. Interdigitation can be better understood by using an example of four images used to form a composite image with a material having at least three lenticules. In this example, line 1 from each of the four images is in registration with the first lenticule; line 2 from each of the four images is in registration with the second lenticule; etc. Each lenticule is associated with a plurality of image lines D or an image line set (See FIG. 1C), and the viewer should see only one image line of each set with each eye for each lenticule. It is imperative that the image line sets be registered accurately with respect to the lenticules, so that the proper picture is formed when the assembly is viewed. However, this is difficult to achieve.
For example, one method of conventional recording of the interdigitated image lines requires recording of the interdigitated image lines on a recording material contained on the substrate layer C and then attaching the substrate layer C to the upper layer A, with the recorded image lines D in precise alignment to the lenticules B to yield the desired image structure. The precise alignment of the specific lenticules with the desired image line set during the attachment of the recording material to the lenticular overlay (i.e., the upper layer A) is difficult to achieve. The imprecise alignment results in a degraded image quality.
Sometimes, the composite image is written to a separate planar sheet, thereby forming an image sheet, which is then aligned and bonded to the back of the lenticular array. For example, U.S. Pat. No. 5,492,578 discloses a method and an apparatus for aligning an image sheet with a lenticular array by bending and stretching of the image sheet relative to the lenticular array while monitoring the alignment during the bonding operation. The technique requires much manual manipulation and a flexible media for the image sheet. U.S. Pat. No. 5,479,270 discloses a method and apparatus for aligning a lenticular array to a separate image sheet that uses a video camera and Moire techniques.
Conventional recording of composite images has been accomplished with a stereoscopic image recording apparatus that uses optical exposure. A light source, such as a halogen lamp, is projected through an original image, via a projection lens, and the light rays are transmitted through the lenticules and focused on the substrate layer of the lenticular array. The composite image is exposed on a recording material of the substrate layer as interdigitated image lines. However, the quality of the composite image is degraded by aberration introduced by the lenticular.
Contact print exposure from a composite image negative to a radiation sensitive layer coated on the back of and in alignment with a lenticular array has been disclosed in U.S. Pat. No. 5,729,332. The disclosed alignment method makes use of multiple video cameras and detectors as well as of special reference grid structures fabricated into the lenticular array and of similar grid structures on the image negative. Analysis of Moire fringes produced by the combination of the grid structures is used to achieve alignment between the lenticular array and the image negative.
Recording of the composite images by scanning exposure to the back of lenticular arrays is also known. U.S. Pat. No. 5,539,487 "Method and Apparatus for Recording Stereoscopic Images and Lenticular Recording Material Used Therefor" by S. Taguchi and S. Igarashi discloses a method of directly exposing a light sensitive recording layer (typically of silver halide emulsion) coated on the backside of a lenticular array. The exposure device utilizes three optical wavelengths which are either scanned or CRT (full image) projected to achieve full color. Post processing is necessary to develop the image as in most silver halide films. The pitch measurement technique disclosed in U.S. Pat. No. 5,539,487 teaches the use of a light emitter cooperating with a light detector. The light emitter and the light detector are positioned just beyond opposite edges of the lenticular array such that the light path between the emitter and detector is parallel to the plane of the lenticular array and the light beam traverses parallel to the long axes of the lenticules. The lenticular array is translated in a direction perpendicular to the long axes of the lenticules. Thus, the light emitted by the emitter and propagated toward the detector can be obstructed or transmitted as this light alternatively strikes the edge of a lenticule or propagates along the valley between alternate lenticules. The signal generated by the detector is used to provide information about the pitch of the lenticular array. However, the resultant signal modulation is relatively low, making it difficult to accurately determine array pitch. In another embodiment this patent discloses the use of a position marker. This marker is provided on the lenticular array at a site outside of the image recording region. This requires that the lenticular array be oversized and necessitates trimming to remove the region containing the marker. This increases the cost of production and wastes portions of the lenticular arrays. This patent is silent with respect to whether or how the lenticular array is rotationally aligned with respect to image lines.
The publication "Development of Motion Image Printer," by H. Akahori, Kenji Iwano, K. Ikeda, Y. Fukui, K. Nobori, K. Kayashima, IS& T 50th Annual Conference Proceedings, page 305, discusses a printer for printing stereoscopic images using a thermal head and thermal dye transfer in registration with the back side of a lenticular array, forming an integral stereoscopic image. The lenticular array must be heated in order to achieve the proper pitch of the lenticules for accurate registration of the image lines with the correct lenticules.
U.S. Pat. No. 5,279,912 disclosed the writing of a composite image by direct scanning exposure of a radiation sensitive layer that is coated on the back of a lenticular array. An observer would see a three-dimensional image by looking at the lenticular array. The disclosure teaches a registration technique that uses a separate light beam of a wavelength different from those that are scanned over the radiation sensitive layer to form the composite image. U.S. Pat. No. 5,279,912 does not disclose an automatic method for angular alignment of the composite image with respect to the lenticular array.
European patent publications 0 596 629 A2 and 0 659 026 A2 disclose the use of a scanning laser beam to thermally transfer colorant from a donor sheet to receiver layer on a lenticular array, for the purposes of providing three-dimensional images. Patent publication 0 596 629 A2 discloses the use of a pre-objective lens two-axis scanner and the use of special structures on the lenticular array outside of the image area. This requires trimming of the lenticular array after the printing of image lines, in order to eliminate the portions (of the lenticular array) that have these structures. Patent publication 0 659 026 A2 discloses the use of detector arrays to achieve registration of a printed composite image to the lenticules. The detector array senses the position of the writing laser beam as it exposes the donor while the writing laser beam moves along the fast-scan axis (which, in this case, is perpendicular to the long axes of the lenticules). Some of the light passes through the absorbing donor sheet as well as any previously deposited colorant and is imaged by the appropriate lenticule to a position on the detector array that gives information on the location of the focused spot (in the plane of the donor sheet) with respect to the lenticular array. This information is used to modulate the writing laser beam to give the appropriate transfer of colorant at the appropriate position on the receiver. Disadvantages of this registration method are its complexity and low light levels or possibly no light delivered to the detectors, unless the transferred colorants are diffusing to the laser light or unless the diffusing layer is attached to the lenticular array. Also, although the two European patent applications discuss the possibility that the fast-scan axis could be parallel to the long axes of the lenticules, these patent applications are silent with respect to how to achieve the required rotational alignment of the fast-scan axis with the long axis of the lenticules.